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Old 14-11-2006, 03:20   #1
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Will Gelcoat adhere to Epoxy?

Will Gelcoat adhere to Epoxy? ~ by Bruce Niederer
As stolen from http://www.SailboatOwners.com

Q. I have heard that gel coat will not adhere well to epoxy. Can you confirm this?

A. There is a common misconception, fueled by some gelcoat manufacturers and by some expensive failures in the field, that gelcoat will not bond to epoxy. Polyester resin bonds poorly in a secondary (mechanical) bonding situation which consequently makes epoxy the resin of choice for repairs. How can one be squared with the other?

The answer is surprisingly simple - gelcoat does bond to a properly cured and prepared epoxy surface. There are a couple issues to be aware of to have success making this repair. There are three situations that cause gelcoat to not cure over epoxy... all related to the hardener chemistry. Epoxy hardeners are basically a blend of amines, which can terminate the chain reaction of the radical molecule that is the basis of polyester (and vinylester) cure chemistry. So by carefully mixing, curing, and preparing of the epoxy so that there are no unreacted amines to interfere with the gelcoat cure, gelcoat bonds quite well to epoxy.

The first situation is undercured epoxy. Gelcoat applied to undercured epoxy will be in contact with unreacted amines and the cure will be halted.

The second situation is if the epoxy is mixed off ratio so that it is hardener rich, again leaving unreacted amines free to interfere.

Third is the issue of amine blush, commonly called blush. Blush is a surface phenomena that is a reaction of the amine molecules at the surface with the carbon dioxide in the air. It forms easiest in the presence of moisture, so working in cool, humid environments will maximize the formation of blush. Any amine hardener has the potential to blush, but it can be minimized by careful choices of amines in the formulation. In fact, WEST SYSTEM 207 Special coating hardener is one of the lowest blushing hardeners on the market and still maintains structural properties on par with our other hardeners. Regardless of chemistry, blush is very easily dealt with because it is water soluble. A simple wash with clear water removes the blush. No soap, no solvents. Then sand that washed surface with 80 grit paper to provide the gelcoat with sufficient key so it won't run. Be sure to use non-air inhibited gelcoat that has a paraffin wax added. Gelcoat is applied over epoxy on a routine basis everyday in boatyards that are aware of these issues.
Bruce Niederer, Technical Services West System Inc. http://www.westsystem.com/

See also the articles at:
http://www.epoxyworks.com/indexprojects.html
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Old 14-11-2006, 03:51   #2
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But....but...... this is contrary to everthing I thought I "knew" ???

Rick in Florida
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Old 14-11-2006, 04:23   #3
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Quote:
Originally Posted by rickm505
But....but...... this is contrary to everthing I thought I "knew" ???
Rick in Florida
Which is why I posted it - to rectify a common misconception in the body of conventional wisdom; as erroneously posited in Don Casey’s Quick Tip #29: Polyester or Epoxy Resin?
http://www.boatus.com/boattech/TipResins.htm

See also:
“Polyester over Epoxy” ~ By Jeff Wright
http://www.westsystem.com/ewmag/22/polyester.html
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Old 14-11-2006, 04:55   #4
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Major f##ck up if you don't get it right. Have wanted to do this on a few projects but never had the guts.

Interestingly ATL composites, the Australian " West " distributor, talked about having an epoxy compatible Gelcoat that was a lot more expensive than polyester gelcoat.

Dave
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Old 14-11-2006, 05:52   #5
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Yes, it is probably quite possible to get polyester to bond to epoxy. But I have seen enough failures by competent people who I would ordinarily trust to "get it right" that I have never tried it myself and always recommend that an expensive experiment not be carried out.

Besides, why would you want to put an inferior coating (gelcoat) on top of a superior coating (epoxy) as long as UV is not a factor.

Richard
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Old 14-11-2006, 07:09   #6
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Quote:
Originally Posted by seaclusion
Besides, why would you want to put an inferior coating (gelcoat) on top of a superior coating (epoxy) as long as UV is not a factor.
You would want to do this when the gelcoat is a finishing step and the repair needs to match the surrounding area.

I have had very good success applying gelcoat over epoxy. In fact, never thought much about it, even though I have always known the facts surrounding the different chemistries and lesser adhesion properties of polyesters. It really is quite simple: follow the epoxy mixing ratios exactly, allow the proper time for cure, wash down the area after cure, sand the area, and apply the gelcoat. The first two steps should be done regardless of your plans for finishing.

If one was applying gelcoat over cured polyester, it would also be a mechanical bond (although brushing on some styrene might reactivate the surface a bit and provide some chemical bond).

Mark
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Old 14-11-2006, 11:34   #7
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Correct in all aspects Mark. (not suggesting I know more, just reinforcing your comments)
One point to make very clear, Gelcoat is no better at adhering than say any paint. All these coatings are Adhering, NOT chemicaly bonding to the cured Epoxy. Only Epoxy will chemicaly bond to Epoxy, PROVIDING the original layer has not fully cured. Once fully cured, the next coat of Epoxy is also adhering only. To fully cure Epoxy it can actually take weeks and sometimes months depending on temperature.
If you want to ensure the Epoxy is fully cured before applying Gelcoat, a heater in the area, ENSURING that a temp of no more than 50C is achieved, for a 24hr period will allow the epoxy to cure close to fully.
But a second process is required. As already suggested, Sanding to provide a "key" for the gel is essential. Just as paint also requires that "key".

Why is Gelcoat used over Epoxy?? well it is for several reasons.
Ease of application and finish. Usually this process is used in production manufacturing of hulls. A Hull mould is sprayed with the Gelcoat first. The Gelcoat is easier to release from a mould than Epoxy is. Plus it totaly eliminates the need for finishing the hull and painting at a later date. So a great saving can be achieved in production manufacturing.
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Old 14-11-2006, 13:10   #8
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Colemj,

You can get a chemical bond between gelcoat (polyester ortho or iso) and polyester substrate by using a polyester resin for laminating that does not contain wax. Polyester resins (and gelcoats for that mater) that have wax in them keep free oxygen from the surface and therefore allow the surface to fully cure. If the resin does not have wax, the the free oxygen in the air will inhibit the cure. The suface will not fully cure and remain tacky to the touch. In fact, my 1973 Gulfstar still has tacky surfaces hidden away behind trim pieces. Any polyester based material layed on top will form a chemical as well as a mechanical bond with the substrate. This is how multiple layers of a fiberglass part are laminated one on top of another and the resultant structure is considered a homogeneous monocoque structure.

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Old 14-11-2006, 13:20   #9
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Wheels,

Gelcoat is sprayed on the inside of a mold so that the following layers of fiberglass and polyester resin will stick to it and form a monocoque structure. If epoxy were sprayed first, the bond between it and the following layers of polyester would be only mechanical in nature and prone to delamination.

Epoxy can be sprayed into a female mold provided that epoxy resins are used for the following fiberglass lay-up. Here in the states it is done quite often. My company has built many specialized parts using epoxy resins and we have found them to be easier to pull the parts because of the tougher surface epoxy forms. Seems to stick less to the mold surface.

Richard
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Old 14-11-2006, 18:56   #10
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I am not a big fan of gelcoat over epoxy, I am not a big fan of gelcoat period. I have made more repairs to boats with gelcoat over polyester, gel over epoxy, that I have finally just settled with awlgrip over epoxy and I havent had any problems yet other than an awlgrip to awlgrip bond. I also dont believe in gelcoat used under the waterline in ANY situation. Say what you want but my observation is that there are a lot of people to offer different opinions but those of us who have done a ton of work on boats have seen all the related gel coat problems and fixed many of them and know that gelcoat (mainly below the waterline) is a real pain and just isnt worth the headaches.

I have a 31 year old Coronado that some claim to be a poor quality boat, and ugly as mud on a fence post but I removed most of the gellcoat on the bottom and did a barrier coat with poxy and I hauled her out 3 weeks ago after 2 years in the hot water of the Florida keys there was not blister one. Not even a pea sized blister. I have a hard red tracer coat over the epooky and black ablative over the tracer coat. great thing about this setup is ZERO sanding. The powerwash removed all the ablative and it is ready to be painted again.

The main point is the hull was in perfect condition. I didnt have to fix anything nor did I need to do any sanding. Just repaint the ablative. HELLO do you know how nice that is????

Putting gelcoat over epoxy is taking a perfect substrate and fouling it up with a product prone to problems when used on boats below the waterline. above the waterline.. go for it, but awlgrip over epoxy is sooo much easier and in my book... just the way to go.

PS I have read all the stuff about using gellcoat over epoxy and what I was read was produced by the gelcoat maker.... I have never come across a good gelcoat job yet that didnt blister.... the proof is in the pooxky

As far as the Coronado... she is a bute' but it took $100,000 in custom work to make her what she is today!
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Old 14-11-2006, 19:15   #11
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Seaclusion, what you said is exactly what I meant. When I said Gel over Epoxy, I meant it being the external coat you see once removed from the mold.
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Old 14-11-2006, 21:50   #12
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Quote:
Originally Posted by classycoro
... PS I have read all the stuff about using gellcoat over epoxy and what I was read was produced by the gelcoat maker.... I have never come across a good gelcoat job yet that didnt blister.... the proof is in the pooxky
...
FWIW: The links I posted, were all from WEST System, the Epoxy producer.
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Old 14-11-2006, 23:41   #13
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Ummmm, a point needs to be made very clear here. Blisters are not caused by Gelcoat. Removing Gelcoat and using a Barrier coat does not always mean Blisters will not return. In fact, there are some incedences where blisters have become worse after a barrier coat has been applied.
It is important to note that the problem layer is the Polyester laminate under the gelcoat. The gelcoat is the "semi-permeable membrane" that allows the Osmosis to occur. But it is the fault of the resin under the gelcoat that traps the water, creates the chemical change and results in a liquid thats molicules are too big to fit back through the membrane and producing the pressures that were large enough to create the blisters.
A comment such as "I have never come across a good gelcoat job yet that didn't Blister" is IMO a gross over exageration. I can show you literaly thousands of permanently emersed hulls that have never had any issues with blisters. Much of that is due to the later years of Vynlester Resin use and the use of Epoxy resins. But the Gelcoats are all the same.
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Old 15-11-2006, 01:17   #14
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Epoxy is good for repairs as it doesnt shrink. I did a repair on my old boat and didn't use epoxy and it later shrunk and the borders of the repair cracked. I ground it out and started again using epoxy with a gelcoat topping after it had cured and no problem.
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Old 15-11-2006, 11:19   #15
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Hmmmm, what did you use the first time Darryl. Cause Poly shouldn't shrink either.
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